The system refers to an alignment system for multiple colour ink jet printheads in a dot matrix printer comprising a fixed structure, a scanning carriage for supporting the printheads, which is movable with respect to the fixed structure in a first horizontal direction, and an electronic control unit comprising a function for timing the emission of ink droplets by the heads.
Colour ink jet printers both of the xe2x80x9cthermalxe2x80x9d type and of the xe2x80x9cpiezoelectricxe2x80x9d type are widely known, said printers being provided with a plurality of monochromatic heads (typically three or four) which are physically and mechanically identical to one another, but contain different inks (typically corresponding to the primary colours blue, yellow and magenta, and in some cases black); each head has a large number of nozzles for emitting ink droplets (for example, three hundred, but the current trend of technology is towards even larger numbers) which are arranged at uniform intervals in one or more parallel rows, with a corresponding number of emission elements for producing the ink droplets which are selectively ejected through the nozzles.
As is known in the latest technology, ink jet printheads of the xe2x80x9cthermalxe2x80x9d type comprise a semiconductor (usually silicon) substrate or chip having, formed on it, using known technologies, both the components which more specifically relate to ink jet technology (the emission resistors) as well as the power drivers for driving them and the logic for selecting the individual emission resistor to be driven; for the first group of components normally thin-film technology is used, while for the second group LDMOS (lateral double diffused MOS) technology and for the third group CMOS technology are used, these latter technologies being used in the simplified version with the least number of process steps and masks, so as to satisfy only the functional requirements of the bubble ink jet heads.
The relative positioning precision of the nozzles with respect to one another on an individual head is very high, since the nozzle-carrying plate is made as one piece and the active part of the head is formed on a single silicon chip, all of which using micro photolithographic techniques which ensure a high degree of mechanical precision. The same cannot be said about the positioning precision with which the chip is assembled on the container body of the head, the latter being in turn mounted on the scanning carriage of the printer, such that final alignment of the nozzles on the various monochromatic heads (necessary for obtaining a good print quality, in particular in high definition mode, as is well known to those skilled in the art) may be obtained only by means of additional operations involving operational alignment of the heads, to be performed in a more or less automatic manner, directly on the printer, with consequent difficulties of a practical and cost-related nature.
The object of the present invention is that of defining a system for obtaining operational alignment, both in the horizontal direction (scanning direction) and vertical direction (interlinear feed direction), of the printheads of a colour ink jet printer having multiple monochromatic heads, with the necessary precision for high-quality high-definition colour printing.
The object of the present invention is obtained by the alignment system for multiple colour ink jet printheads and associated printhead with built-in optical position detector, characterized by the steps of providing printheads comprising a built-in opto-electronic position detector, for detecting a first misalignment of the heads in the first horizontal direction and a second misalignment of the heads in a second vertical direction, substantially perpendicular with respect to the first horizontal direction, the heads also comprising a plurality of nozzles arranged at uniform intervals in at least one row parallel to the second vertical direction; providing an illumination device integral with the fixed structure for producing a light spot focused on the opto-electronic detector; detecting the first misalignment and the second misalignment by means of the combined use of the built-in opto-electronic position detector and the illumination device; compensating for the first misalignment of the heads in the first horizontal direction, by means of a variation of the timing of the emission of ink droplets, and compensating for the second misalignment of the heads in the second vertical direction by means of a vertical displacement of the heads.
The system according to the invention is based on the availability of printheads comprising at least one opto-electronic device which forms an optical position detector of the four-quadrant type, integrated on the chip itself of the head, i.e. manufactured during the course of the same production process, using standardized technologies for the production of integrated semiconductor circuits, with which, based on the common silicon substrate, the other components necessary for operation of the head itself, such as the emission resistors, the selection circuits, the drivers and the connection conductors, are made.
In this way the integrated opto-electronic device forms an optical position detector which is aligned with the nozzles with photolithographic precision and by means of which it is possible to detect automatically both the horizontal position and the vertical position of each individual monochromatic head mounted on the scanning carriage; the system according to the invention uses the position detection operations thus performed in order to make the appropriate corrections needed to compensate for the geometric alignment errors detected, by means of the electronic control unit of the printer.
The horizontal alignment errors are corrected by suitably delaying or advancing the timing of emission of the ink droplets by the various monochromatic printheads with respect to the instant determined on the basis of the theoretical position of the head itself and the speed of displacement of the carriage; the vertical alignments errors are corrected, on the other hand, by suitably modifying the physical position of the heads (for example, by vertically displacing the head by a suitable amount by means of a piezoelectric micromotor) or else (assuming a maximum misalignment one half pitch) by dispensing with use of the nozzles of each head located outside a common alignment band.
Another object of the present invention is that of defining a system for adjusting both the horizontal and vertical mechanical alignment of the seats supporting the printheads of an ink jet colour printer having multiple heads, with the precision necessary for high-definition colour printing, during testing of the printer itself or during a maintenance operation, using a sample printhead.
A further object of the invention is that of defining a system for adjusting both the horizontal and vertical mechanical alignment of the chip+flat cable assembly subgroup with respect to the container of a printhead for a colour ink jet printer having multiple heads, during construction of the head itself.
Another object of the invention is that of defining a system for adjusting both the horizontal and vertical mechanical alignment, relative to one other, of two or more chips which make up a multiple printhead (able to print in a single pass a strip with a height typically greater than one inch) for particular applications (for example, postal franking).
Another object of the invention is that of defining a printhead provided with an optical position detector integrated on the same semi-conductor chip and using the same technologies used to manufacture the emission elements and the logic and power circuitry.
The abovementioned objects are achieved by means of a system for operational alignment of the monochromatic printheads of a colour ink jet printer provided with multiple heads, and by means of a printhead with a built-in optical position detector, which are characterized as defined in the main claims.
These and other objects, characteristic features and advantages of the invention will emerge clearly from the following description of a preferred embodiment thereof, provided by way of a non-limiting example, with reference to the accompanying drawings.